Many complex and expensive high power systems use mixed heat exchangers to dissipate the waste heat. A mixed heat exchanger can be defined by the use of one medium to cool another. For example, air or hydrogen gas may be used as a cooling medium for the power system components and then the resulting heat contained in the air or hydrogen is cooled by a liquid. Gas or hydrogen by itself is easy to control, is clean, and is electrically insulative. In areas where environmental requirements dictate, a closed loop, self-contained cooling system is used to keep out containments and moisture. In a self-contained cooling system, hydrogen gas is often selected as it offers low windage and ventilation losses due to its low density. Ventilation losses are proportional to the gas density. Additionally, hydrogen has a much higher thermal conductivity that air. Therefore in a closed system, hydrogen gas makes a good choice in cooling high powered electrical systems, such as large electric generators. Waste heat in a closed system must be efficiently extracted to the outside. Liquid, on the other hand, conducts heat much better than gas, allowing for a greater heat transference.
Mixed heat exchangers use both gas and liquid arrangements to capitalize on the advantages of both. Gas is -circulated through a system, such as the internal workings of a power generator, cooling desired sub-systems until the heat capacity of the gas is reached. The warm gas is then typically regenerated by passing in close contact with a cooler liquid, such as water. As the gas passes in close contact with the water, the heat is transferred to the water, cooling the gas so that is may once again by cycled through the system.
Though there are various types of contacts between the gas and water, it is usually desired that there be no direct physical contact between the two. This is so the gas remains dry and does not pick up any water vapor or droplets. In fact, water contamination is one of the primary problems in dealing with mixed heat exchangers.
FIG. 1 illustrates a simplified example of how gas in a mixed heat exchanger may be cooled by water flow. Heated gas 2 that has cooled a different part of the system enters a heat exchanger area 4 that typically comprises some bends or spirals. Cooler water 6 flows in close proximity to the gas 8, but is separated from it by some form of barrier 10. Since the nature of the heat exchanger relies on the ability to effectively transfer heat from the gas to the water, it is necessary that the barrier be as thin as possible. Cracks or fissures form in the barrier 12 which causes a leak in of water into the gas flow 14.
In order to determine if there are water leaks, current methods involve using float valves with the gas portion of the mixed heat exchangers. When water pools in places, buoyant martial floats on the water, indicating a leak. This technique only works for large leaks, and relatively small leaks that disperse minute water droplets are not detected. Another technique is to measure the dew point inside of the gas portions of the mixed heat exchangers. Though this will allow for the eventual detection of small leaks, it takes a long time before a small leak increases the dew point of the large volume of gas in the heat exchanger to register on the dew point monitors.
What is needed is a leak detector for mixed heat exchangers that can reliably determine if there is a leak, as well as the approximate location of the leak in the exchanger.